Physics Colloquium: Mixing in Interstellar Turbulence

Liubin Pan
Harvard Clay Postdoctoral Fellow
Harvard-Smithsonian Center for Astrophysics
Title : Mixing in Interstellar Turbulence
Mixing of heavy elements in the interstellar medium occurs in the presence of a supersonic turbulent velocity field. I will first discuss simulation results for the general physics of passive scalar mixing in isothermal supersonic turbulence. Turbulent mixing is driven by a cascade process similar to that of the velocity field. Using six simulated turbulent flows with the Mach number, M, in the range from 1 to 6, we find that, when normalized to the flow dynamical time, the mixing timescale increases with M for M <3, and becomes essentially constant for M > 3. This result suggests that compressible modes are less efficient in enhancing mixing than solenoidal modes. I will also discuss the Mach number dependence of other passive scalar statistics, such as the probability distribution function (PDF), the power spectrum, the structure functions, and intermittency. The second topic of my talk is motivated by the pollution process of primordial gas in high-redshift galaxies, which controls the transition from Pop III to Pop II star formation in these galaxies. The transition occurs as heavy elements produced by the first stars pollute the surrounding gas and increase its metallicity beyond a critical value, Z_c (10^-8 < Z_c <10^-5). Using both theoretical and numerical methods, we investigate how the mass fraction of pristine fluid elements with Z < Z_c evolves in compressible turbulent flows. Our theoretical approach is based on the PDF method for turbulent mixing, and using several existing PDF models, we derived evolution equations for the pristine fraction. We find that a class of PDF models, named the self-convolution models, provide successful fits to the simulation results for the pristine fraction. Our results can be used to set up a subgrid model for large scale simulations for the pollution of primordial gas in early galaxies.
Bio: Liubin Pan is currently a Harvard Clay Postdoctoral Fellow at the Harvard-Smithsonian Center for Astrophysics. He earned his Ph.D. in 2008 from the University of Texas at Austin, and was subsequently a postdoctoral research fellow at Arizona State University.